You can perform a clean install from a Windows 7 upgrade version however you will have to have a valid previous copy of Microsoft Windows XP or Microsoft Windows Vista (Windows 7 RC or EVAL will not qualify) installed to validate the upgrade and activate the product key. Please note that when migrating from Windows XP to Windows 7 you will not have an "in place upgrade" option. You will however have the option to select "custom" install when prompted. The Windows 7 install process will then copy all of your data in "My Documents" over to a Windows.old folder within Windows 7 itself. All applications and documents stored in other locations will have to be reinstalled / transferred manually.
For more information on the Windows 7 Upgrade, please go here: http://windows.microsoft.com/en-US/windows7/Installing-and-reinstalling-Windows-7
For additional assistance with the migration of Windows XP to Windows 7, please go here: http://technet.microsoft.com/en-us/windows/ee150430.aspx
Microsoft Windows Client Team